U.S. patent number 8,570,890 [Application Number 13/000,851] was granted by the patent office on 2013-10-29 for mobile radio communications device and related method of operation.
This patent grant is currently assigned to NEC Corporation. The grantee listed for this patent is Michael Nosley, Weili Ren. Invention is credited to Michael Nosley, Weili Ren.
United States Patent |
8,570,890 |
Ren , et al. |
October 29, 2013 |
Mobile radio communications device and related method of
operation
Abstract
A mobile station establishes UL synchronization by the use of
downlink preamble messages, and by monitoring timing drift within
the downlink signalling and by updating the UL synchronization
value responsive to the magnitude of the timing drift exceeding a
threshold value. Tx power values can also be updated as a function
of the updated synchronization value.
Inventors: |
Ren; Weili (Berkshire,
GB), Nosley; Michael (Berkshire, GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Ren; Weili
Nosley; Michael |
Berkshire
Berkshire |
N/A
N/A |
GB
GB |
|
|
Assignee: |
NEC Corporation (Tokyo,
JP)
|
Family
ID: |
39707889 |
Appl.
No.: |
13/000,851 |
Filed: |
June 18, 2009 |
PCT
Filed: |
June 18, 2009 |
PCT No.: |
PCT/JP2009/061549 |
371(c)(1),(2),(4) Date: |
January 26, 2011 |
PCT
Pub. No.: |
WO2010/001789 |
PCT
Pub. Date: |
January 07, 2010 |
Prior Publication Data
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|
|
|
Document
Identifier |
Publication Date |
|
US 20110116407 A1 |
May 19, 2011 |
|
Foreign Application Priority Data
|
|
|
|
|
Jul 2, 2008 [GB] |
|
|
0812089.1 |
|
Current U.S.
Class: |
370/252; 370/337;
370/516 |
Current CPC
Class: |
H04B
7/2681 (20130101); H04W 56/00 (20130101) |
Current International
Class: |
H04J
1/16 (20060101); H04J 3/16 (20060101) |
Field of
Search: |
;370/252,337,516,347 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1326623 |
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Dec 2001 |
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CN |
|
1607787 |
|
Apr 2005 |
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CN |
|
1859675 |
|
Nov 2006 |
|
CN |
|
1 130 793 |
|
Sep 2001 |
|
EP |
|
2357401 |
|
Jun 2001 |
|
GB |
|
2413036 |
|
Oct 2005 |
|
GB |
|
2007015828 |
|
Feb 2007 |
|
WO |
|
2007088633 |
|
Aug 2007 |
|
WO |
|
2008136488 |
|
Nov 2008 |
|
WO |
|
Other References
IEEE Standard for Local and metropolitan area networks, Part 16:
Air Interface for Fixed Broadband Wireless Access Systems, IEEE
802.16e, 2005. cited by applicant .
Japanese Office Action for JP2010-550970 mailed on Oct. 10, 2012.
cited by applicant .
International Search Report for PCT/JP2009/061549 mailed Oct. 13,
2009. cited by applicant .
IEEE Standard for Local and metropolitan area networks, Part 16:
Air Interface for Fixed Broadband Wireless Access Systems, IEEE Std
802.16. XP-002544590, 2004, pp. 1-16, 152-177, 343-345, 477-480,
620-622, 637-640. cited by applicant .
WiMAX Forum, Mobile System Profile Release 1.0 Approved
Specification, Revision 1.4.0, May 2, 2007. cited by applicant
.
Chinese Office Action for CN 2009801242645 dated on Feb. 28, 2013
with Partial English Translation. cited by applicant .
Taiwanese Office Action for TW098121066 dated on Feb. 8, 2013.
cited by applicant.
|
Primary Examiner: Lee; Andrew
Claims
The invention claimed is:
1. A method of controlling uplink (UL) synchronisation in a mobile
station of a radio communications network, and including the steps
of monitoring timing drift within downlink (DL) signalling and
updating an uplink synchronisation value to obtain an update uplink
synchronization value, in response to the said timing drift
exceeding a threshold value; wherein the update uplink
synchronisation value is determined on the basis of the sum of a
previous synchronisation value and twice the aforementioned
magnitude of the said timing drift in the DL signalling.
2. A method as claimed in claim 1, wherein the threshold value is
transmitted by way of Downlink Channel Descriptor (DCD)
signalling.
3. A method as claimed in claim 2, wherein the threshold value is
determined at a base station within the radio communications
network and responsive to at least cell size, cyclic prefix length
and measurement accuracy.
4. A method of determining an update synchronization value for
uplink synchronisation in a mobile radio communications network and
having regard to a current synchronisation value, including the
step of determining timing drift within downlink signalling and
wherein the update synchronisation value comprises the sum of the
said current synchronisation value and twice the magnitude of the
said timing drift.
5. A method as claimed in claim 1, wherein the timing drift is
determined on the basis of DL preamble measurements.
6. A method of controlling a Tx power value in a mobile station in
a radio communications network, including the step of monitoring
timing drift within downlink (DL) signalling, updating an uplink
synchronization value, and determining the updated uplink
synchronisation value on the basis of the sum of a previous
synchronization value and twice the magnitude of timing drift in
the signalling and wherein the updated Tx power value comprises a
function of the said update uplink synchronisation value.
7. A method as claimed in claim 6, wherein the Tx update value is
determined whenever the uplink synchronisation value is
updated.
8. A method as claimed in claim 1 and provided within a WiMAX
system.
9. A method as claimed in claim 1, where the network signalling
arises in Time Division Duplex mode.
10. A mobile radio communications device for use within a mobile
radio communications network and arranged for controlling uplink
synchronisation values, and including: a section which monitors a
downlink timing drift value; a section which updates the uplink
synchronisation value to obtain an update uplink synchronization
value, in response to the magnitude of the downlink timing drift
value exceeding a threshold value; and a section which determines
the update uplink synchronisation value on the basis of the sum of
a previous uplink synchronization value and twice the said
magnitude of the downlink timing drift.
11. A mobile radio communications device for use in a mobile radio
communications network and arranged for determining an update
synchronization value for uplink synchronisation and including
means for determining the magnitude of downlink timing drift and
wherein the update synchronisation value is determined on the basis
of the sum of a previous synchronisation value for the uplink
synchronization and twice the magnitude of the said timing
drift.
12. A device as claimed in claim 10, and including means for
determining an updated Tx power value as a function of the update
uplink synchronisation value.
13. A device as claimed in claim 12 and arranged for updating the
Tx power value when the uplink synchronisation value updates.
14. A device as claimed in claim 10, and arranged such that the
timing drift is determined on the basis of downlink preamble
measurements.
15. A method as claimed in claim 4, wherein the timing drift is
determined on the basis of DL preamble measurements.
16. A method of controlling a Tx power value in a mobile station in
a radio communications network, including the step of determining
timing drift within downlink signalling and determining an update
uplink synchronisation value on the basis of the sum of a current
synchronization value and twice the magnitude of the said timing
drift, and wherein the updated Tx power value comprises a function
of the said update uplink synchronisation value.
17. A method as claimed in claim 16, wherein the Tx update value is
determined whenever the uplink synchronisation value is
updated.
18. A method as claimed in claim 4 and provided within a WiMAX
system.
19. A method as claimed in claim 4, where the network signalling
arises in Time Division Duplex mode.
Description
This application is the National Phase of PCT/JP2009/061549, filed
Jun. 18, 2009, which is based upon and claims the benefit of
priority from UK patent application No. 0812089.1, filed on Jul. 2,
2008, the disclosure of which is incorporated herein in its
entirety by reference.
TECHNICAL FIELD
The present invention relates to a mobile radio communications
device such as any form of User Equipment (UE) of a mobile radio
communications network, and to a method of operation of the same
and in particular which seeks to improve operational
characteristics thereof.
BACKGROUND ART
For mobile radio communications devices such as UE handsets
operating within a mobile radio communications network, it can
prove important to control various operational characteristics in
an attempt to optimise performance of the handset within the
network. For example, synchronisation of the UE with a network Base
Station (BS) and/or the control of transmission power within the UE
comprise important characteristics determining the overall
efficiency of operation of the UE.
Techniques are currently known for attempting to maintain the
required synchronisation and/or appropriate transmission power
levels for the UE and commonly involve a signalling exchange
between the UE and the BS. However, this has a disadvantageous
effect on power requirements and on the signalling load within the
network. In particular, the power requirements of the UE increased
and limitations on the possible deployment of power-saving features
such as "sleep mode" can lead to further operational
inefficiencies.
DISCLOSURE OF INVENTION
The present invention seeks to provide for a mobile radio
communications device, and related method of operation, having
advantages over known such devices and methods.
According to one aspect of the present invention there is provided
a method of controlling UL synchronisation in a mobile station of a
radio communications network, and including the steps of monitoring
timing drift within downlink signalling and updating the uplink
synchronisation value responsive to the said timing drift exceeding
a threshold value.
The required synchronisation can therefore advantageously be
maintained without requiring UL signalling. The aforementioned
threshold value can be transmitted by way of Downlink Channel
Descriptor (DCD) message signalling.
Further the said threshold value can be determined at a' base
station within the radio communications network and responsive to
at least cell size, cyclic prefix length and measurement
accuracy.
In particular, the update uplink synchronisation value can be
determined on the basis of the sum of the current synchronisation
value and twice the magnitude of the said timing drift in the DL
signalling.
According to another aspect of the invention there is provided a
method of determining an update value for UL synchronisation in a
mobile radio communications network and having regard to a current
synchronisation value, including the step of determining timing
drift within downlink signalling, and wherein the update
synchronisation value comprises the sum of the said current
synchronisation value and twice the magnitude of the said timing
drift.
Advantageously, the timing drift is determined on the basis of DL
preamble measurements.
It will be appreciated that the method outlined above can be
provided within a WiMAX system. Further, the method can prove
particularly applicable for a communication system operating in
Time Division Duplex mode.
Turning now to another aspect of the invention, there is provided a
method controlling a Tx power value in a mobile station in a radio
communications network, including the step of determining uplink
synchronisation as outlined above and wherein the updated Tx power
value comprises the sum of a previous power value and a function of
the said synchronisation.
It should be appreciated that the method of Tx power control can
include the method of controlling UL synchronisation outlined above
and such that the Tx update value arises whenever the uplink
synchronisation value is updated.
Yet further, the present invention can provide for a mobile radio
communications device for use within a mobile radio communications
network and arranged for controlling uplink synchronisation values,
and including means for monitoring a downlink timing drift value
and means for updating the uplink synchronisation value responsive
to the magnitude of the downlink timing drift value exceeding a
threshold value.
The device preferably includes means for determining the updated
uplink synchronisation value on the basis of the sum of the
previous value and twice the said magnitude of the downlink timing
drift.
Still further, the invention can provide for a mobile radio
communications device for the use in a mobile radio communications
network and arranged for determining an update value for uplink
synchronisation and including means for determining the magnitude
of downlink timing drift and wherein the update synchronisation
value is determined on the basis of the sum of the previous uplink
synchronisation value and twice the magnitude of the said timing
drift.
The aforesaid device can include means for determining an updated
Tx power value as a function of the updated uplink synchronisation
value.
Also, the device can be arranged for updating the Tx power value
when the uplink synchronisation value updates.
Of course the device can be arranged such that the timing drift is
determined on the basis of downlink preamble measurements.
As will be appreciated, the invention provides for autonomous
maintenance within an MS of the UL Timing_Offset value and which
therefore advantageously removes the need for periodic ranging
procedures and so can reduce signalling overhead and radio resource
usage.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described further hereinafter, by way of example
only, with reference to the accompanying drawings in which:
FIG. 1 is a timing diagram for downlink preamble transmissions
arising in signalling between a network base station and mobile
station;
FIG. 2 is a flow-diagram illustrating the operation of the present
invention in relation to a mobile station embodying the same;
and
FIG. 3 is a schematic diagram of a mobile station comprising a UE
handset arranged according to one embodiment of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The following discussion of an embodiment of the present invention
is based upon IEEE standards (IEEE 802.16-2004 and IEEE
802.16e2005) relating to WiMAX systems requiring in particular that
a UE such as a Mobile Station (MS) that remains connected with a
network BS should maintain Up Link (UL) synchronisation and also
update UL transmission power so as to compensate appropriately for
its mobility within the network.
As will be appreciated from the known standards, initially the UL
synchronisation, also referred to as "Time_Offset", and the MS
transmission power, commonly identified as Tx power, are acquired
by the MS and delivered from the BS by way of Initial Ranging
signalling during network entry and initialisation.
On the basis of the arrival of a Ranging signal from the MS, the BS
can measure the round-trip propagation delay of the signal which is
then known to be equivalent to the Timing_Offset that the MS needs
to apply for appropriate UL transmission. Also, based upon the
signal strength of the received Ranging signal, the BS can
calculate the required Tx power adjustment that should be applied
in the MS for appropriate UL transmissions.
Once so determined, the BS can send such values for the
Timing_Offset and Tx power to the MS in its Ranging Response
signalling.
It is also noted that the Ranging signal may experience multi-path
fading in the UL channel such that its arrival time at the BS may
suffer delay spread and its received signal strength at the BS may
fluctuate.
Further, it is known that in Orthogonal Frequency Division Multiple
Access (OFDMA) a Cyclic Prefix (CP) is added to each OFDM symbol to
collect a fading delay spread, and in order to prevent inter-symbol
interference. An additional link budget margin is also reserved to
accommodate received signal strength fluctuation caused by such
fading. As noted later, and due in particular to fading delay
spread, the required Timing_Offset accuracy within OFDMA systems
can be defined as one-quarter, or at least one-half, of the CP
(Cyclic Prefix) duration.
After the Initial Ranging signalling, appropriate Timing_Offset and
Tx power levels are maintained via Periodic Ranging until the MS
disconnects from the BS. In addition to such Periodic Ranging, the
normal UL data transmissions can also be employed to update
Timing_Offset and Tx power adjustments, if such transmissions occur
prior to the next scheduled Periodic Ranging signal.
The ongoing adjustment of the Timing_Offset and the Tx power are
generally required so that the MS transmissions remain aligned with
the BS with timing allocations within the BS, such as the receive
frame, and also that the transmissions are received within the
appropriate reception power thresholds. The Periodic Ranging
currently employed in an attempt to maintain the appropriate
Timing_Offset and Tx power values is generally controlled by way of
a Periodic Ranging timer. For example, in WiMAX systems, the
Periodic Ranging timer is defined within the MS and, upon
expiration of the timer period, is arranged to indicate that the MS
has not been given an opportunity to transmit to the BS for a
predetermined period of time such that Periodic Ranging should be
triggered. The aforementioned predetermined period of time
comprises a time period beyond which it is assumed that the
previous Timing_Offset and Tx power values for the UL transmission
will no longer be valid and must be re-set.
As an example, it is known from the WiMAX Forum, Mobile System
Profile Release 1.0 Approved Specification, Revision 1.4.0,
2007-05-02 that the following parameters arise or can be
employed:
Frame duration=5 ms
Bandwidth=10 MHz
Sampling rate=28/25
Cyclic Prefix=1/8
Fast Fourier Transform (FFT) size=1024
From the above parameters, and through calculation that the CP
duration is in the order of 11.4 .mu.s, the Timing_Offset accuracy,
generally set at one quarter of the CP duration, can be considered
as in the order of 2.85 .mu.s and this corresponds to a one way
propagation distance of 427 m. Then, taking into account potential
movement of the MS, for example if moving within a vehicle at
maximum motorway speed of 70 mph, the MS may well move beyond the
propagation distance within a time of 13.7 s. Thus, to take account
of this, the time-out value of the Periodic Ranging timer within
the WiMAX MS should not exceed 13.7 s. Indeed, if the WiMAX system
is to be employed with an MS moving at an even higher speed such
time-out value will of course have to be set at a much smaller
value within the Periodic Ranging timer. As clarified below, such
an increase in frequency of Periodic Ranging signalling triggered
by the timer emphasize the disadvantages that arise through use of
this known procedure for maintaining UL synchronisation and Tx
power values.
In particular, the use of Periodic Ranging for updating UL
Timing_Offset and Tx power values introduces a control signalling
overhead, generally in the form of Medium Access Control (MAC)
management message exchanges. Further, a subset of Code Division
Multiple Access (CDMA) codes will also have to be reserved for the
Periodic Ranging signalling and so this removes such codes which
could otherwise be used by the purposes within the system, for
example within an initial ranging so as to further reduce the
probability of signalling-conflict. Such known Periodic Ranging
also reduces the effectiveness of power saving features such as
"sleep mode" operation since the Periodic Ranging will require the
MS to wake from its sleep mode. In particular, CDMA-based Periodic
Ranging can also undesirably extend the MS availability Interval in
order to await the reception of the Ranging response message, in
order to identify the appropriate Ranging opportunity for
sending/resending CDMA-based Ranging requests.
As noted above, such disadvantages are particularly emphasized if
the Periodic Ranging time-out value is reduced so as to compensate
for potential high speed movement of the MS within the network.
Turning now to FIG. 1, there is provided a timing diagram for the
transmission and receipt of downlink signalling at a MS and which
illustrates, in particular, the potential drift in DL preamble that
can arise due to MS mobility. Such drift is advantageously
determined, and employed, in accordance with this particular
embodiment of the present invention.
As will be appreciated, while MS continues to move, the magnitude
of the preamble drift keeps changing. In the illustration of FIG.
1, there is shown the downlink signalling 10 as transmitted at a
BS, and the same signalling as received 12 at a MS.
As will be appreciated, each of the signals 10, 12 employs a series
of interlaced DL-subframes A, and UL-subframes B wherein each
DL-subframe comprises a preamble portion 14 and main body portion
16. As will be appreciated, at time t.sub.1 there is a particular
value of Timing_Offset which represents the difference between the
time of transmission of the preamble 14, and its receipt at the MS.
However, due to the mobility of the MS, there is a drift in the
Timing_Offset such that, for later subframes, for example at time
t.sub.2 there has been a change in the delay at which the preamble
of the DL-subframe is received at the MS, this difference, i.e. the
preamble drift that occurs between the points in time t.sub.2 and
t.sub.1 is identified as .DELTA.t.sub.p.
According to this embodiment of the present invention, the MS is
advantageously arranged to monitor the magnitude of the preamble
drift .DELTA.t.sub.p and compare the same with a threshold value.
The threshold value corresponds to the maximum variation that the
BS and MS can tolerate for correct operation. Should the threshold
value be exceeded by a magnitude greater than the threshold value
of .DELTA.t.sub.p, then the Timing_Offset will have to be updated
within the MS. Likewise, the MS timing reference against the DL
preamble can also be updated.
In general operation of a MS such as that receiving the signalling
as indicated at 12 in FIG. 1, at the time of entry into a WiMAX
network the MS will perform an Initial Ranging procedure to acquire
the correct initial Timing_Offset and Tx power adjustment
values.
If the WiMAX system is operating within Time Division Duplex (TDD)
mode, the advantages of the present invention are particularly
enhanced since the UL and DL channels will share the same frequency
band and the effects of the multi-path propagation environment
between the BS and MS will impact the UL and DL transmissions in
the same way at any particular time. That is, since the UL and DL
channels use the same frequency band, signal propagation in the UL
or DL channel will experience the same multi-path fading and delay
spread profiles, and the path-loss patterns will also be the same
for both the UL and DL transmissions.
It should be appreciated that the UL Timing_Offset of the MS can be
determined on the basis of the distance of the MS from the BS, and
is generally equal to a round-trip propagation delay in the MS and
BS.
It is also noted that an active MS frequently measures the DL
preamble for channel estimation and other purposes, and even while
the MS is in a "sleep mode", it is also arranged to measure the DL
preamble regularly whenever it wakes-up during a listening
window.
Through the Initial Ranging procedure, the MS will also have
acquired the initial UL Timing_Offset and initial Tx power levels
which can be subsequently employed as follows.
On the basis of the regular measurement of the DL preamble within
the MS, the MS can readily perceive DL preamble timing drift
.DELTA.t.sub.p as illustrated in FIG. 1 and, in particular, detect
the magnitude of the DL timing drift .DELTA.t.sub.p accumulated
over the time period t.sub.2-t.sub.1.
The new Timing_Offset value for update purposes can then be
determined from
Timing_Offset.sub.new=Timing_Offset.sub.previous+2*.DELTA.t.sub.p
Thus, and as noted above, when the magnitude of .DELTA.t.sub.p
exceeds the aforementioned threshold value, a determination of the
new Timing_Offset value required can readily be made, and
subsequently employed for communication between the MS and BS.
The threshold value against which the magnitude of the preamble
drift .DELTA.t.sub.p can advantageously be broadcast by way of DCD
messages. The actual value can be determined at the BS taking into
account at least cell size, cyclic prefix length and measurement
accuracy.
In addition to the employment of the updated Timing_Offset value
that can be readily determined within the MS on the basis of the DL
preamble measurements, the invention can also advantageously allow
for UL Tx power adjustment updates so as to compensate for
path-loss changes.
That is, the MS again has read access to the initial Tx power
instruction BS via the Initial Ranging procedure discussed above
and the updated Tx power value can readily be determined on the
basis of a function of the updated Timing_Offset value determined
as described above. Thus, it is required that the Timing_Offset
value to be updated, the Tx power value can likewise be updated and
as based on a particular propagation loss formula which can readily
embodied as a function of the updated Timing_Offset value such as:
Tx_power.sub.new(dB)=f.sub.x(Timing_Offset.sub.new)
If widely used COST 231-Hata Propagation Model is adopted, then
Function f.sub.x can be derived as:
Tx_power.sub.new(dB)=46.3+33.9*log(f)-13.82*log(Hb)-a*(Hm)+[44.9-6.55*log-
(Hb)]*log(Timing_Offset.sub.new*c/2) Where
a(Hm)=[1.1*log(f)-0.7]*Hm-[1.56*log(f)-0.8], Hm=height of the
antenna of mobile station Hb=height of the antenna of base station
f=carrier frequency c=speed of light
The Tx power adjustment is generally provided so as to compensate
for changes in power-loss value changes.
Thus, it will be appreciated that the illustrated embodiment of the
present invention allows for the advantageously accurate estimation
of updated Timing_Offset and Tx power values based on the
assumption that the MS can measure and record the DL preamble
timing drift in an accurate manner.
Autonomous maintenance of the UL synchronisation and Tx power
values can therefore be maintained without requiring the specific
periodic signalling arising in the prior art.
The invention therefore allows for advantageously simple
implementation of a synchronisation and power control scheme and
one embodiment is illustrated by way of the timing diagram found in
FIG. 2.
Here, the signalling between a MS such as a UE handset 18 of a
WiMAX system and a BS 20 of the same system is illustrated and
commences with initial broadcast of DCD messages 22 from the BS 20
to the MS 18 and which can contain a preamble drift threshold value
against which the magnitude of the measured preamble drift of the
receive DL signals is compared.
The initial ranging response signalling 24 is delivered from the BS
20 to the MS 18 and contains the initial Timing_Offset and Tx power
values etc. As required, the initial preamble drift, Timing_Offset,
and Tx power values are set at 26, 28 and 30 respectively and prior
to repeated performance of DL preamble measurement procedures
32.
Thus, within the repeated procedure 32, the DL preamble from the BS
20 is measured so as to detect the timing drift value at 34 and, at
36, the timing drift value is updated as required. However, at 38
it is determined whether or not the magnitude of the preamble drift
is greater than the threshold value contained in said DCD message
22. If the threshold is not exceeded, then the repeated procedure
32 continues to its initial conclusion.
If, however the magnitude of the preamble drift exceeds the
threshold value, then a new Timing_Offset value is determined at 40
on the basis of the sum of the previous offset value and twice the
magnitude of the timing drift .DELTA.t.sub.p.
Likewise, and as discussed above, the primary updating of the
Timing_Offset value 40 can be achieved along with a Tx power update
at 42 and the timing drift itself can be reset such as at step
44.
As noted, the procedure 32 is then repeated at each measurement of
the DL preamble and so, again, allows for control of the UL
synchronisation and Tx power values without requiring initiation of
signalling from the MS 18.
While the proposed technique of the present invention can
advantageously autonomously maintain the Timing_Offset value that
is usable for the whole period for the active connection between
the MS and BS it will of course be possible, as an option, to also
provide for provision of a T4 timer so as to allow for periodic
ranging but in accordance with a far greater time out value, for
example, in the order of a few minutes, rather than a few seconds,
as employed in the current art.
Thus, with reference to FIG. 2, an optional provision of
time-trigger periodic ranging 46 can be provided if required.
As will therefore be appreciated, the present invention
advantageously remove the need, or greatly reduces, the reliance on
periodic ranging and thereby removes or drastically reduces the
related signalling overhead. Also, periodic ranging CDMA codes can
now be released for other purposes and which can be employed to
enhance the general system performance and that can lead to
simplification of the implementation of the MS and its related
BS.
Also, the effectiveness of other operational characteristics, such
as power saving "sleep mode" can be improved.
As will therefore be appreciated, the invention presents a new
technique for the MS to autonomously maintain UL synchronisation
and transmission power adjustment after it obtains initial UL
Timing_Offset and transmission power level from the BS via Initial
Ranging procedure. The proposed technique is based on the fact that
signal propagation in the UL and DL channels experiences the same
multi-path fading and delay spread profile when the system operates
in TDD mode, where the UL and DL share the same frequency band. The
MS then makes use of its regular DL preamble measurements and
detects DL timing drift accumulated over time, which is equal to
half of the UL Timing_Offset variation. TDD mode ensures that the
MS autonomous calculations on UL Timing_Offset and transmission
power adjustment are appropriately accurate so that the MS can
maintain UL synchronisation for a much longer time. Adoption of the
technique can remove the necessity of Periodic Ranging defined in
IEEE 802.16 standard for maintaining the active MS UL
synchronisation, and thereby remove the related signalling
overhead, reduce radio resources usage and improve power saving
performance in Sleep Mode.
Turning now to FIG. 3, there is provided a simplified schematic
diagram of a MS comprising UE handset 48 arranged to embody the
present invention.
Thus, as will be appreciated, the handset 48 contains standard
signal reception/transmission circuitry 50, processor unit 52 and
an arrangement 54.
The processor unit 52 includes an element 52A, arranged for
determination of the preamble drift of the receiver DL signalling,
and the comparison of the same with a threshold value and along
with functionality for determining an updated Timing_Offset and/or
Tx power values as required in accordance with the present
invention.
The present invention advantageously provides a departure from the
known art in so far as the DL preamble signal can be employed to
measure the DL timing drift accumulated over a period of time and
during which no UL transmissions take place.
The MS can then autonomously calculate the UL Timing_Offset
variation caused by its own mobility within the network, and so
arrive at an appropriate Timing_Offset update as required.
Although the technique is proposed for WiMAX TDD mode, this is not
a limiting feature of the invention since it can be employed in
other modes such as, for example, WiMAX FDD mode where UL and DI
channels use different frequency bands. Since frequency bands are
well separated in UL and DL directions, signal propagation in UL
and DL channels may have slightly different fading delay spread
profiles, thus the autonomous calculation of Timing_Offset might
not be as accurate as in WiMAX TDD mode, but nevertheless still
readily employable within the invention.
The proposed technique can also be applied to other OFDMA based
systems, particularly 3GPP LTE. As will be appreciated LTE is
designed to support high mobility (up to 350 km/h) and the CP
duration is shorter than in WiMAX and so this can result in a more
demanding accuracy requirement for the autonomous calculation of
Timing_Offset, and Tx power.
* * * * *